Thermionic cathode heaters



y 1959 B. J. GREEN 2,885,334

THERMIONIC CATHODE. HEATERS Filed Dec. 28, 1954 //v v /vron B. J. GREENATTORNEY United States Patent O 2,885,334 THERMIONIC CATHODE HEATERSBernard Jeffrey Green, London, England, assignor to InternationalStandard Electric Corporation, New York, N.Y., a corporation of DelawareApplication December 28, 1954, Serial No. 478,028

Claims priority, application Great Britain January 1, 1954 3 Claims.(Cl. 204-181) The present invention relates to heaters for thermioniccathodes. Heaters for thermionic cathodes are normally coated withalumina to ensure adequate cathode-heater insulation. In the past it hasbeen commonly accepted that a desirable heating coating should be smoothand certainly free from any cracks. We have found, however, that,particularly with fine heater wire, in order to avoid brittleness of theheaters, a smooth and dense coating should be avoided, it beingpreferred that the coating should, on the contrary, be intentionallymade so as, in use, to have a surface crazed with small fissures.

According to the present invention, therefore, there is provided athermionic cathode having a heater coated with alumina, the surface ofthe alumina being intentionally crazed with small fissures.

Cathode heaters of the coil type are usually. coated either by sprayingor electrophoretically. Straight or faggot type heaters are usuallydipped or dragged. Although, over a very long time, electrophoreticmethods of coating have been tried out, for various reasons they havenot been entirely successful, and, in consequence, for coiled heatersspraying is more popular, in spite of the great waste of coatingmaterial involved. The main fault with electrophoretic coating up to thepresent time has been its extreme embrittling effect on tungsten. Ifthis eifect could be eliminated, or at least minimised, theelectrophoretic method of coating has great advantages, vlz:

1) It gives a coating which provides a consistent value ofcathode-heater insulation.

(2) The thickness of the coating can be accurately controlled withinfiner limits than is possible with spraymg.

(3) The coating process is very simple, little skill being required toproduce good heaters.

(4) The economy of material is very great compared with spraying, inwhich some 99% of the coating is wasted in the spray booth, air ductsetc.

Alumina exists in three main forms: a-alumina, which has a specificgravity of 3.8 and is characterised by a triagonal crystal structure;B-alumina with a specific gravity of 3.3 and a hexagonal crystalstructure; and 'y-alumina having a cubic structure and a specificgravity of 2.8.

In the past electrophoretic coatings were based on a suspensionconsisting mainly of u-alumina in a suitable liquid, the suspensioncontaining little or no binder. The coated wire, in the case ofcontinuously coated heaters passed direct into drying and sinteringovens. In the case of unit heaters such as double helical coils whichwere dipped into the electrophoretic bath, an external nitro-cellulosebinder was applied, usually after drying, but sometimes the binder wasincluded in the coating suspension. This binder supported the coatinguntil sintering was completed.

In both cases a very smooth closely packed coating of big densityalumina was obtained with a hard porcelainlike character. This coatingbecame even harder during processing and life of the valve, due to thecoalescing of the particles aided by a certain amount ofrecrystallisation of the alumina. The result was a coating which becamea solid rod of high mechanical strength. In the case of fine tungstenWires the coating was actually strong enough to fracture the tungsten byleverage across the already enlarged crystal boundaries. Thisbrittleness became progressively less with increasing core diameter.

The three forms of alumina mentioned above are related in that, on heattreatment 'y-alumina can be converted into 19- and on further heatingthe )8- into aalumina. With a view to overcoming the brittleness ofheaters coated with tat-alumina we use a less dense alumina as part ofthe coating, which coating shrinks on processing, producing a network offine cracks. This allows for free expansion and contraction of theheater core without affecting the electrical properties of theinsulator.

Accordingly, the present invention further provides the method ofmanufacture of a heater for a thermionic cathode in which the heater iselectrophoretically coated with alumina containing eight parts by weightof 'yalumina for every eight to twelve parts of a-alumina, resulting inthe fully processed cathode having a heater with an insulated coatingwhich is crazed with small fissures. The preferred proportion of 'y toon alumina is such as to result in the mean specific gravity of themixed alumina being 3.5.

The invention will be further described with reference to theaccompanying drawings in which:

Fig. 1 is a drawing taken from a photomicrograph of a portion of aheater according to the present invention after electrophoretic coatingand before processing and Fig. 2 is a drawing taken from aphotomicrograph of a portion of heater manufactured according to thepresent invention after removal from a valve which has been fullyprocessed.

The 'y-alumina for use in coatings of the present invention is firsttreated with hydrochloric acid, as known in the art, to condition thecharge carrying properties of the particles.

The preparation of the electrophoretic suspension is preferably carriedout in the following manner:

900 millilitres of N-butyl alcohol, 600 millilitres of a 5% solution ofnitrocellulose in butyl acetate, together with millilitres of N-butylacetate and 9 millilitres of N-butyl phthalate are introduced into aball-mill and then 1000 grams of a-alumina, as normally supplied in thetrade for coating radio valve heaters, and 800 grams of 'y-alumina,treated as above, are added. Finally, a sufiicient amount (of the orderof 40 millilitres) of a 25% solution of aluminum nitrate in butylalcohol is added to the mix to provide, in the final suspension, a pHvalue of between 2.5 and 3.0. The mix is then ball-milled for 24 hours,after which it is kept under constant operation until required for use,agitation being maintained in the electrophoretic bath.

In general I have adopted the method of the present invention for thecoating of pre-formed helical heaters of shapes which are not convenientfor coating by a drag process and also for very fine wire heaters whichwould not stand up to spraying, even if this were otherwise desirable.The heaters are suitably supported in the coating bath and after coatingthey are immediately washed in methylated spirits. The spirit removesthe suspension dragged out from the coating bath but does not attack theelectrophoretically deposited layer. The heaters are then dried in awarm air blast and sintered.

The appearance of the heater wire after coating but before sintering isdepicted in Fig. 1 of the accompanying drawings, and shows an even butrelatively coarse covering.

During the subsequent processing of the heater, the following changes inthe composition of the alumina take place.

800 C. 'y-alumina cubic crystals 1000 C. B-alumtna hexagonal crystalsa-alumina triagonal crystals The increase in specific gravity causes avolume shrinkage in the coating which results in stresses being set up.The stresses eventually cause cracks to appear in the coating. Thechange, however, is a continuous process, dependent on atemperature-time factor. Stages at which the changes occur are not yetfully determined,

but they are not normally complete until after the heater I has beenused for some time in a completed discharge device.

Fig. 2 shows the change in appearance of heater wire according to theinvention on removal from a thermionic valve which has been normallyprocessed and aged.

It will be seen that, while the surface of the coating has 41 scribedabove in connection with specific embodiments, and particularmodifications thereof, it is to be clearly understood that thisdescription is made only by Way of example and not as a limitation onthe scope of the invention.

What we claim is:

1. The method of manufacture of a thermionic cathode comprising thesteps of electrophoretically coating the cathode heater with aluminafrom a bath containing eight parts by weight of 'y alumina for eacheight to twelve parts of a alumina, drying the coating, heating thecoating between 800-1000 C. for conversion of the 'y alumina ultimatelyinto or alumina, whereby the resultant mixture has an increased specificgravity which causes a volume shrinkage of the coating and crazingthereof with small fissures.

2. The method of manufacture according to claim 1 in which saidresultant mixture of 'y and or. aluminas has a mean specific gravity of3.5.

3. The cathode of claim 1, wherein the alumina coating comprises amixture of substantially like compositions by weight of 'y-alumina ascubic crystals and a-alumina as triagonal crystals with a mean specificgravity range between 2.8 and 3.8.

References Cited in the file of this patent UNITED STATES PATENTS1,942,879 Riddle Jan. 9. 1934 1,975,870 Schrader Oct. 9, 1934 2,092,815Shaw Sept. 14, 1937 2,164,913 Goodchild July 4, 1939 2,442,864 SchneiderJune 8, 1948 2,734,857 Snyder Feb. 14, 1956

1. THE METHOD OF MANUFACTURE OF A THERMIONIC CATHODE COMPRISING THESTEPS OF ELECTROPHORETICALLY COATING THE CATHODE HEATER WITH ALUMINAFROM A BATH CONTAINING EIGHT PARTS BY WEIGHT OF 2 ALUMINA, WHEREBY THERETO TWELVE PARTS OF A ALUMINA DRYING THE COATING, HEATING THE COATINGBETWEEN 800*-1000* C, FOR CONVERSION OF THE V ALUMINA ULTIMATELY INTO AALUMINA , WHEREBY THE RESULTANT MIXTURE HAS AN INCREASED SPECIFICGRAVITY WHICH CAUSES A VOLUME SHRINKAGE OF THE COATING AND CRAZINGTHEREOF WITH SMALL FISSURES.